The present invention relates to a process useful for blending two miscible fluids of widely differing viscosities at a high concentration of the low viscosity component to form a homogeneous blend of the two fluids.
It is known that blending low viscosity additives, such as plasticizers and solvents, with a high viscosity fluid, such as a polymer melt, is a difficult problem. The low viscosity additive, if added in significant quantities, often channels through the higher viscosity fluid, resulting in incomplete blending. To mitigate this problem, frequently a mechanical mixing device employing one or more rotating shaft(s) is used in the early stages of the process to mix the low viscosity fluid into the high viscosity fluid rapidly. This mechanical mixing step has the drawbacks of increasing the process temperature due to mechanical energy input and requiring leak-proof seals around the rotating shaft in the case when one of the components to be blended is flammable or an environmental hazard. These seals present a potential safety or environmental problem since they have a tendency to rupture with wear.
Static mixers, also known as motionless mixers, have also been employed in an attempt to prevent this channeling from occurring. The static mixing elements divide the fluid flow into thin streams or striations creating increased surface area between the striations. With increasing mixer length, the additive is more finely distributed and then dissolved. U.S. Pat. No. 6,179,458 (Albers et al.) discloses the use of a mixing device wherein mechanical mixing elements are driven on a rotating shaft in a process for mixing high concentrations of a low viscosity fluid into a high viscosity fluid. To accomplish uniform mixing, the low viscosity component is added at different axial locations along the process stream with rotating mixing elements after each injection point to maintain the high viscosity fluid as the continuous phase of the mixture. Upon blending by the mechanical mixing elements whereby a homogeneous solution is formed, the solution is forwarded to a series of static mixers. Prior to each of these static mixers is an additional low viscosity fluid injection point for dilution of the solution to the desired final concentration. This system for mixing generally results in long mixer lengths and high pressure drops across the mixing elements of the system.
Static mixers have been employed to blend fluids of significantly different viscosities. European Pat. No. 472 491 B (assigned to Sulzer Chemtech Ltd.) discloses a mixing device which includes static mixing elements useful for blending a low viscosity fluid or gas and a highly viscous fluid, and an admixing device useful for introducing the low viscosity fluid or gas additive into the highly viscous fluid at a single axial location. In one disclosed embodiment, the mixing device is divided into two adjoining mixing columns, a premixer and a main mixer. The admixing device includes an opening and a nozzle for introducing the low viscosity fluid or gas into the highly viscous fluid. The orifice for combined flow is composed of a converging inlet and diverging outlet with design based on the relative flow rates and allowable pressure drops. Amounts of up to 4-6% or more of the low viscosity additive are disclosed as possible to be dissolved in the highly viscous fluid with the use of the device.
U.S. Pat. No. 5,176,448 (King et al.) discloses a static mixing device useful for blending a small amount of a low viscosity fluid with a much larger amount of a high viscosity fluid, utilizing a circular injection head biscuit placed within a conduit, the biscuit having a plurality of openings therethrough. The openings have mixing elements for inducing a rotational angular velocity to the fluid stream. The low viscosity additive is pumped through a nozzle in the biscuit.
U.S. Pat. No. 4,753,535 (King) discloses a static mixing device useful for blending or premixing a small amount of a low viscosity fluid with a much larger amount of a high viscosity fluid, comprising a generally tubular device located within a conduit. The device has an entry port shaped like the frustrum of a cone on its upstream end for the addition of one fluid to the other, and a hollow shaft on its downstream end. Within the hollow shaft are static mixing elements for the blending of the two fluids. A second mixing apparatus may be placed downstream of the device.
Attempts to increase the amount of low viscosity fluid additive to above about 10% in such blends generally result in the low viscosity component channeling through the high viscosity component. When the high viscosity phase is not continuous in laminar or turbulent flow, it becomes difficult to generate shear stress high enough for mixing or blending to occur. As a result, staged injection of the low viscosity fluid and/or additional time under shear stress become necessary so that the fluid components may be uniformly blended.
An improved process is needed by which increased amounts of a low viscosity fluid may be added to and blended with a highly viscous fluid such as a polymer at commercially attractive rates and process conditions.
The present invention is a process for forming a uniform homogeneous blend of two fluid components having a large difference in viscosity, the process comprising:
a) pumping a high viscosity fluid component into a first conduit and maintaining the high viscosity fluid component at a temperature and a pressure sufficient to allow a single phase to form;
b) injecting a low viscosity fluid component into the high viscosity fluid component flowing through the first conduit wherein the ratio of the viscosities of the two fluid components is at least 10,000:1 and the low viscosity fluid component is provided in an amount of about 30-90% by weight of the total weight of the low viscosity fluid component and the high viscosity fluid component;
c) forwarding the low viscosity and high viscosity fluid components to a second conduit connected to the first conduit containing a first set of static mixing elements having a length to diameter ratio of at least 18, such that the high and low viscosity fluid components have a shear rate in excess of 0.57 secxe2x88x921;
d) forwarding the low viscosity and high viscosity fluid components to a third conduit connected to the second conduit, the third conduit containing a second set of static mixing elements having a diameter larger than the first set of static mixing elements and having a length to diameter ratio of at least 18, such that the high and low viscosity fluid components have a shear rate in excess of 0.20 secxe2x88x921, whereby a uniform homogeneous blend or solution is formed.